ARSENIC ATTENUATION IN THE HYPORHEIC ZONE : DOES IT ALWAYS DEPEND ON DISCHARGE OF ARSENIC- RICH GROUNDWATER?

In many arsenic-affected regions around the world, e.g., the Bengal Delta Plain (BDP) groundwater arsenic discharges into surface rivers and is attenuated in a hyporheic zone mainly by adsorption on river sediments. These stream sediments subsequently, under favorable conditions, release arsenic to groundwater and hence are considered as significant recycled sources of groundwater arsenic. Although, enriched As concentration in groundwater was reported earlier from Chianan Plain (CP), SW Taiwan, stream sediments and hyporheic zone were not studied so far to assess their potential as a possible source/sink.

Present study at the CP involves systematic collection and measurement of shallow subsurface sediments and few selected source rocks (including major and trace elements, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, sequential extractions, magnetic susceptibility studies, and electron microprobe analysis), pore water, and river water sampling and analyses for major and trace ions and stable isotopes.

Our study shows that arsenic concentration in river sediments (including hyporheic zone) generally corresponds to the world average concentration in soil and upper crustal abundance. The ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, magnetic susceptibility data and comparison with source rocks suggest arsenic in sediments is not related to phyllosilicates. Sequential extraction results indicate that arsenic is mostly bound in amorphous and crystalline FeOOH. Although, a clear delineation of hyporheic zone is possible through selective major ions and stable isotopes in pore water, no anomalous enrichment of solid phase arsenic in river sediments (compared to source rock) or depletion of aqueous arsenic in pore water was observed across the zone excepting manganese which (300-600 mg/kg) is largely enriched in river sediments. This suggests that arsenic attenuation takes place deeper than in the studied hyporheic zone and arsenic discharge into rivers is limited. In contrast, Mn is oxidized later on the discharge pathway and across the hyporheic zone. Therefore, arrest of arsenic in stream sediment at CP seems to depend on change in redox conditions not only across the hyporheic zone, but also along the entire discharge pathway unlike BDP, where arsenic accumulation occurs predominantly across the hyporheic zone.